1. Field of the Invention
This invention relates to an apparatus for heating semiconductor bodies during the processing thereof as in processing by temperature gradient zone melting.
2. Description of the Prior Art
In the manufacture of semiconductor devices, it is often necessary to alter or tailor the conductivity type of a body of semiconductor material. This alteration is achieved by distributing atoms of a conductivity modifying dopant in a selected region or selected regions of the body. Frequently, techniques employed to achieve that distribution require the heating of the semiconductor body.
Recently, it has been discovered that a preferred technique of altering the conductivity of a semiconductor body is the technique of temperature gradient zone melting. Early teachings of temperature gradient zone melting are found in U.S. Pat. No. 2,813,048 issued to W. G. Pfann and in a book by Pfann entitled Zone Melting (John Wiley and Sons, Inc., 1966). More recent applications of temperature gradient zone melting are disclosed and claimed in U.S. Pat. Nos. 3,899,361 and 3,899,362 to Thomas R. Anthony and Harvey E. Cline and assigned to the assignee of the present invention. The aforementioned U.S. Patents to Anthony and Cline are incorporated herein by reference. As taught in the aforementioned book by Pfann, temperature gradient zone melting involves the migration of a melt of a first material through a body of a second material under the influence of a temperature gradient maintained across the second material and finally a recrystallization of the melt after migration. In the application of temperature gradient zone melting to the production of semiconductor devices, the first material usually comprises a metal such as aluminim, tinaluminum alloy or gold-antimony alloy while the second material comprises a semiconductor material such as silicon, germanium or the like.
In his co-pending U.S. Patent Application Ser. No. 578,736, filed May 19, 1975, and now U.S. Pat. No. 4,001,047, entitled heating Apparatus For Temperature Gradient Zone Melting, John Boah discloses tungsten filament quartz lamps capable of emitting infra-red radiation as being suitable for establishing a temperature gradient in a temperature gradient zone melting process. This application is assigned to the assignee of the present invention and is incorporated herein by reference. Boah also discloses a radiant heater marketed by Research, Inc. of Minneapolis, Minn. which employs a planar array of such quartz lamps as being suitable. However, such a radiant heater is deficient in a number of respects. In the commercial manufacture of semiconductor devices, it is desirable to process as many devices at a time as possible. Moreover, it is desirable to employ a heat source wherein failure of one or more heating elements will not adversely affect the desired temperature gradient to a degree requiring the discarding of the semiconductor bodies being processed when the failure occurred. In a temperature gradient zone melting process, it is therefore desirable to employ a planar array of radiant heating elements of as large an overall area as possible. Prior art radiant heaters are capable of accommodating quartz lamps disposed in a planar array of only limited area. Exemplary is the aforementioned heater marketed by Research, Inc. which employs six GE 3200T, 3/1 CL Quartz Infrared Lamps marketed by General Electric in an array measuring approximately 18 inches by approximately 3 inches. Planar arrays of radiant heating elements of greater areas are required for the economic commercial production of semiconductor devices by temperature gradient zone melting.
Critical to the successful implementation to temperature gradient zone melting in the manufacture of semiconductor devices is the maintenance of a unidirectional temperature gradient across the semiconductor material. That is, for any desired direction of migration of the conductivity altering dopant through a body or wafer of semiconductor material, a temperature gradient across the body must be established in a direction as close to the desired direction as possible. The existence of temperature gradients in any other directions (lateral or oblique to the major surfaces of the wafer) will cause the migration of the dopant in these other directions thereby causing the resulting zone to be irregular or misdirected. Such zone geometries adversely affect the performance of the semiconductor device. Due to the spacing of lamps within a planar array thereof, temperatures may vary by as much as 40.degree. C over distances as small as 1 cm in the area of illumination of the array. These temperature irregularities contribute to the aforementioned lateral or oblique temperature gradients. Constructing an array of suitable size for commercial production of semiconductor devices from a plurality of prior art heaters would significantly contribute to these lateral or oblique temperature gradients due to the relatively wide inter-lamp spacing between lamps disposed on the edges of two adjacent arrays.
Another source of temperature irregularities causing the adverse lateral or oblique temperature gradients described hereinabove is the failure or burn-out of a lamp or lamps within a planar array thereof. Therefore, it may be necessary to halt production of semiconductor devices to allow the servicing and replacement of the burned-out lamp. Such a halt in production would be uneconomical and therefore should be minimized in time duration. Prior art heaters require substantial disassembly for the replacement of a single lamp. In fact, the same disassembly may be required to replace one or a plurality of lamps. This substantial disassembly and the time required therefor, may add significantly to the cost of manufacturing semiconductor devices by temperature gradient zone melting.
In addition to the interlamp spacing, severe temperature irregulariities in the area of illumination of a lamp array will be caused by the construction of the lamps themselves. Direct observation of such an array through solar filters has indicated that these irregularities in temperature are due to the self shadowing effect of the coiled tungsten filament, the shadowing of the disk-shaped filament holders disposed within the quartz envelope and the refractive properties of the quartz envelopes. Prior art quartz lamp heaters employ no means for minimizing these temperature irregularities, which irregularities if of a great enough value, could cause the misdirection of the melt zones thereby adversely affecting the semiconductor device being produced.
Other sources of temperature irregularities and the corresponding lateral and oblique temperature gradients produced thereby are the position of a semiconductor wafer relative to the center of the area of illumination of the lamp array and the finite size of the array. Since each lamp emits infrared radiation over a particular angular range, a semiconductor wafer disposed in the center of the area of illumination will be irradiated by a plurality of lamps. However, a wafer disposed near the edge of the area of illumination will be irradiated by a lesser number of lamps. Therefore, when a plurality of wafers are being processed, the temperature gradients impressed across wafers positioned at the center and edges of the area of illumination will differ causing a non-uniformity in the migration of the melt zones within two such wafers. Moreover, the varying temperatures within the finite area of illumination will cause lateral or oblique temperature gradients to be impressed across the wafers being irradiated.
The dependence of the infrared transmissivity of quartz upon the angle of incidence which the radiation makes with the quartz also produces temperature irregularities in the radiation emitted by the lamps and therefore lateral or oblique temperature gradients resulting from those temperature irregularities. In prior art radiant heaters, each lamp filament must emit to the semiconductor wafers through at least two layers of quartz, one being the quartz envelope of the lamp and the other being a quartz cover plate of an air cooling channel disposed around the lamps. Therefore the transmissivity of the quartz envelopes and coverplate will vary over the angular range of radiation emitted by a lamp causing temperature irregularities and the resulting lateral or oblique temperature gradients over this range.
Therefore, it is an object of the present invention to provide a new and improved semiconductor body heater which overcomes the deficiencies of the prior art.
It is another object of the present invention to provide a new and improved semiconductor body heater capable of irradiating an area of sufficient size for the large scale commercial manufacture of semiconductor devices.
It is another object of the present invention to provide a new and improved semiconductor body heater wherein an individual heating element employed therein may be readily removed and replaced without disturbing any other heating elements.
It is another object of the present invention to provide a new and improved semiconductor body heater wherein temperature irregularities over an area heated by the heater are minimized.